Environmental control systems or facility management systems are employed in office buildings, manufacturing facilities, and other similar buildings, for controlling the internal environment of the facility. The environmental control system may be employed to control temperature, fluid flow, humidity, lighting, boilers, chillers, power, security and similar systems in the internal environment.
For example, in environmental control networks configured to control temperature and air flow, controlled air units such as variable air volume (VAV) boxes and unitary devices (UNT) are preferably located throughout the facility to provide environmentally controlled air to the internal environment. The controlled air is preferably provided at a particular temperature or humidity so that a comfortable internal environment is established and maintained.
The VAV boxes and unitary devices are typically coupled via duct work to a source of conditioned air, known as an air handling unit (AHU). VAV boxes and unitary devices may include a fan or other device for blowing the controlled air. VAV boxes and unitary devices may also include a damper for regulating the amount of the controlled air provided to the internal environment. The damper may be coupled to an actuator, which preferably positions the damper so that appropriate air flow is provided to the internal environment.
In modern systems, a digital controller may typically be associated with at least one of the actuator and the damper. The controller receives information related to the air flow and temperature (known as “controlled variables”) in the internal environment and appropriately positions the actuator so that the appropriate air flow is provided to the internal environment.
The AHU also includes a digital controller which may control the supply of cooled air by regulating the flow of chilled water through a cooling coil. The controller preferably regulates the flow of chilled water to the cooling coil by adjusting the position of a valve based on a feedback signal indicative of the temperature of the air discharged from the coil. The feedback signal is typically generated by a sensor disposed to monitor the controlled variable.
The AHU and VAV controllers use the feedback signals to maintain the controlled variables within certain tolerances of desired levels (known as “setpoints”). For example, the AHU controller attempts to maintain the temperature of the air discharged from the system at a specific level. When the actual temperature of the discharged air deviates from the desired temperature, the controller preferably appropriately adjusts the flow of the chilled water to bring the actual air temperature back in line with the desired air temperature. Thus, if the feedback signal indicates that the actual air temperature is colder than the desired temperature, the controller preferably decreases the flow rate of chilled water to cause the actual temperature of the discharged air to increase. Likewise, if the feedback signal indicates that the actual air temperature is warmer than the desired temperature, the controller preferably increases the flow rate of chilled water to cause the actual temperature of the discharged air to decrease.
An ideal feedback control system would be able to maintain the controlled variable at the setpoint based only on the feedback signal. However, actual feedback control systems may require additional inputs known as control parameters that are used by the controller to determine how to control the system based on the feedback signal and the setpoint. Common control algorithms that make use of such control parameters are proportional (P) control, proportional integral (PI) control, and proportional-integral derivative (PID) control.
With any of the foregoing feedback control strategies, however, it may be difficult to maintain the controlled variable precisely at the desired setpoint for various reasons, including that the appropriate values for the control parameters may change over time as the system is used. For example, the dynamics of a process may be altered by a heat exchanger fouling, an inherent nonlinear behavior, ambient variations, flow rate changes, large and frequent disturbances, and unusual operations status such as failures, startup and shutdown. The process of adjusting the control parameters of a controller to compensate for such system changes is called retuning. If a controller is not periodically retuned, the control response may become poor. For example, the controlled variable may become unstable or oscillate widely with respect to the setpoint. This can result in inefficient operation as well as increase the maintenance costs due to unnecessary wear of the components.
Monitoring the performance of environmental control systems and diagnosing problems therewith have been disclosed in commonly owned U.S. Pat. No. 5,555,195 (“the '195 patent”), U.S. Pat. No. 5,682,329 (“the '329 patent”), and U.S. Pat. No. 7,031,880 (“the '880 patent”). These patents disclose diagnostic systems that may be utilized to analyze the performance of devices in an environmental control system such as an HVAC or VAV box. The diagnostic systems disclosed in these patents advantageously record temperature, air flow, actuator position and other data used in the VAV controllers and generate associated performance indices such as exponentially weighted moving averages (EWMAs). The performance indices may be related to error values, process output values, actuator positions, changes in actuator positions, duty cycles of the actuators, or starts, stops and reversals of the actuators. The calculated and stored performance indices allow building operators to analyze the VAV boxes and controller performance during particular time periods (e.g., commissioning) as well as during the useful lifetimes of the systems.
In addition to monitoring and diagnostic systems such as described above, it is also known to provide alarm/warning systems and data visualization programs to assist building operators with deriving meaningful information from the data that is gathered. However, human operators must typically select the thresholds for alarms and warnings, which may be a daunting task. If the thresholds are too tight, numerous false alarms may be issued. Conversely, if the thresholds are too loose, equipment or system failures can go undetected.
Additionally, it is also known to perform a statistical scaling on performance indicators. As disclosed in the '880 patent, the performance indicators may be received by a system for one or more control applications. The units of the performance indicators are preferably converted into consistent for each type of control application. Subsequently, the units are preferably normalized based on the equipment size. Upon standardizing and normalizing the performance indices, a statistical scaling is performed. A known method for performing such statistical scaling involves determining the standard deviation (z-value) for each performance index. However, the statistical scaling method finds less utility in control systems that use on-off control or staged control outputs.
In view of the foregoing, it would be desirable to provide an improved method and apparatus for conveying measured performance indices to building operators. It would be further desirable to be able to convey and compare performance indicators for control systems with staged-control outputs and proportional outputs on the same scale.